I made a large batch of SMBC today. It turned out really nice and fluffy but it has a little bit of grains in it. Can you please tell me how to fix this batch. I don't want it to go waste

push it though a strainer

I don't know if straining it will help, since the sugar crystals are so small, but I certainly wouldn't throw it away.  If you don't want to use it all over the cake, use it for filling in several cakes.  Likely the graininess will be less noticeable as filling.  I am sure it tastes fine.

well sure just depends on how small the strainer holes and how big the sugar -- but if you use the icing with coconut or nuts it won't be as noticeable -- or candied ginger because it is covered in sugar -- y'know or anything candied like that -- candied citrus peel maybe